interlaced GIF 
An interlaced GIF displays images in two passes of alternating horizontal lines. Depending on which Web browser is being used, interlaced GIFs may produce a venetian-blind effect or simply a blurry or blocky image that gradually sharpens. This presents an advantage for viewers who have slower dial-up connections: Viewers can see the image outlines sooner, so the Web pages appear to load faster than those with noninterlaced graphics. If a browser doesn't support interlaced images, an image will appear as a normal GIF.